docs: add qlty badge

Author: Licoler

README.md

@@ -1,6 +1,6 @@
 # TaskFlow
 
-[![Quality Gate Status](https://sonarcloud.io/api/project_badges/measure?project=Licoler_TaskFlow&metric=alert_status)](https://sonarcloud.io/summary/new_code?id=Licoler_TaskFlow)
+[![Maintainability](https://qlty.sh/gh/Licoler/projects/TaskFlow/maintainability.svg)](https://qlty.sh/gh/Licoler/projects/TaskFlow)
 
 TaskFlow is a full-stack todo app built from a vanilla JavaScript TodoMVC-style frontend and a Node.js API. Users can register, log in, and manage their own tasks through JWT-authenticated API requests backed by PostgreSQL.